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Geological Terms Beginning With "G"

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A black, coarse-grained intrusive igneous rock that is the compositional equivalent of basalt. Composed of calcium-rich feldspars, pyroxene and possibly olivine, but containing little if any quartz. Picture of Gabbro.

Gage Height:

A measured height of water above a reference datum. Frequently used to describe the height of water in a stream, lake, well, canal or other water body.

Gaging Station:

A facility on a stream, lake, canal, reservoir or other water body where instruments are installed to automatically monitor the water. Measurments such as stage, discharge, water temperature and pH are automatically taken and transmitted to hydrologists via satellite, radio or telephone. Measurements from these stations are useful for a wide variety of flood prediction, water management, recreation and navigation purposes.

Gas Field:

The geographic area that is directly above an underground accumulation of natural gas that is commercially viable.

Gathering Pipeline:

A pipeline that carries natural gas between a production well and a main transmission line.

Gathering System:

A network of small pipelines that connect producing wells to the main transmission system.


A study of the time relationships of rock units. Includes methods of both relative and absolute dating.


The science of Earth's landforms, their description, classification, distribution, origin and significance.


A major trough or downwarp of the Earth's crust, in which great thicknesses of sedimentary and/or volcanic rocks have accumulated.

Geothermal Gradient:

The progressive increase of temperature with depth into the Earth.


A hot spring that intermittently erupts a spray of steam and hot water. Caused by the heating of ground water within a confined opening in hot rock.

Glacial Rebound:

A very gradual uplift of Earth's crust that occurs after the weight of a thick continental ice sheet (which produced subsidence) has melted away.

Glacial Striations:

Grooves and scratches on a bedrock surface that were produced by the movement of a glacier. The orientation of the striations gives evidence to the direction of glacial movement.

Glacial Valley:

A valley with a U-shaped cross section that was cut by an alpine glacier.


A thick mass of ice that forms on land from an accumulation and recrystallization of snow significant enough to persist through the summer and grow year by year. There are two basic types of glaciers: 1) valley (or alpine) glaciers that creep downslope under the influence of gravity, and 2) continental glaciers that flow outward from a thick central area under their own weight.


An amorphous (without crystal structure) igneous rock that forms from very rapid cooling of magma. The rapid cooling does not provide enough time for crystal growth.


A coarse-grained, foliated rock produced by regional metamorphism. The mineral grains within gneiss are elongated due to pressure and the rock has a compositional banding due to chemical activity. Picture of Gneiss.


An elongated, downthrown block bounded by two steeply dipping normal faults. Produced in an area of crustal extension.

Graded Bedding:

A rock layer that has a progressive change in particle size from top to bottom. Most common is a sequence with coarse grains at the bottom and fining upwards, which is typically caused by a declining current velocity within the depositional environment.


A coarse-grained, intrusive igneous rock composed primarily of light colored minerals such as quartz, orthoclase, sodium plagioclase and muscovite mica. Granite is thought to be one of the main components of continental crust. Picture of Granite.


Clastic sedimentary particles of any composition that are over 2 mm in diameter.

Gravity Anomaly:

A geographic area where the gravitational attraction is significantly higher or significantly lower than normal.

Greenhouse Effect:

A warming of the atmosphere caused by carbon dioxide and water vapor in the lower portions of the atmosphere capturing heat that is radiated from and reflected by Earth's surface.


A low-grade metamorphic rock that frequently contains green minerals such as chlorite, epidote and talc.

Ground Moraine:

A blanket of till that is deposited during the retreat of a glacier.

Ground Water:

Water that exists below the water table in the zone of saturation. Ground water moves slowly in the same direction that the water table slopes.

Ground Water Recharge Area:

A location where surface water or precipitation can infiltrate into the ground and replenish the water supply of an aquifer.


A seamount with a flat top.

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